Heat-sensitive recording materials are well-known which utilize the color-forming reaction between a colorless or pale-colored leuco dye and an organic or inorganic developer, such that the two chromogenic materials are brought into contact with each other by heating to thereby produce a recorded image. Such heat-sensitive recording materials are relatively inexpensive, and the recording apparatuses therefor are compact and easy to maintain. For these reasons, heat-sensitive recording materials are used not only as recording media for facsimiles and a variety of computers, but also have found a wide range of uses.
Examples of such uses include cash register receipts for POS (point-of-sale) systems, paper for various tickets, etc. With the expansion of POS systems, the uses and usage environments of heat-sensitive recording materials have diversified, and their use under difficult conditions is increasing. Moreover, the printing speed of printers is increasing year by year, so that a demand exists for heat-sensitive recording materials that can record at lower energy. In addition, because heat-sensitive recording materials are also used as receipts, the recorded portions need to exhibit good seal ability and preservability against oils, plasticizers, office stationery, hand creams and the like.
The color-forming reaction in heat-sensitive recording materials comprising, over a support, a heat-sensitive coloring layer principally composed of a leuco dye and a developer is reversible, and therefore the recorded images are known to lose color with time. Such color fading is accelerated by exposure to light, high temperatures, and high-humidity atmospheres, and also proceeds rapidly when in contact with plasticizers, oils and the like, to such an extent that the recorded images become illegible. Furthermore, when such heat-sensitive recording materials are used as paper for parking tickets, they are left in cars, under a severe high-temperature atmosphere especially during summertime, so that the background of the heat-sensitive recording materials becomes colored and loses contrast with the recorded portions (recorded images), and in some extreme cases, the recorded images become illegible.
In order to overcome these problems, methods for improving the preservability of recorded images have been suggested, for example, by forming a protective layer on a heat-sensitive coloring layer, or by adding an epoxy compound to a heat-sensitive coloring layer. None of these methods, however, have yet to provide sufficient improvements in the preservability of recorded images against oils, plasticizers and the like. Providing a protective layer also has a drawback in that the sensitivity is lowered. Moreover, although the problems of color fading by exposure to light, high temperatures and high-humidity atmospheres, and lowered contrast at high temperatures are about to be solved owing to the recent advent of developers providing high recorded-image preservability, the drawback of low recording sensitivity still remains.
The use of 4-allyloxy-4′-hydroxydiphenylsulfone as a developer or a component of a developer is disclosed (see Patent Document 1), and developers that are used together with this developer are also disclosed, such as oligomer-type developers (see Patent Document 2), 4,4′-dihydroxydiphenylsulfone (see Patent Document 3), and 2,4′-dihydroxydiphenylsulfone (see Patent Document 4). Other heat-sensitive recording materials are also disclosed which comprise, in addition to 4-allyloxy-4′-hydroxydiphenylsulfone, an urea-urethane-based compound (see Patent Document 5), p-acetylbiphenyl as a specific sensitizer (see Patent Document 6), or a methylated methylol melamine condensate as a specific preservability improving agent (see Patent Document 7). Furthermore, a heat-sensitive recording material is disclosed which comprises, in addition to 4-allyloxy-4′-hydroxydiphenylsulfone, a dye, a developer, and various preservation stabilizers (see Patent Document 8), or a heat-sensitive recording material which comprises a specific compound without using 4-allyloxy-4′-hydroxydiphenylsulfone (see Patent Document 9). None of these heat-sensitive recording materials, however, have provided satisfactory properties with respect to heat resistance, recording sensitivity and recorded-image preservability.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2001-310561    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-283741    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2004-130539    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2004-195747    Patent Document 5: Japanese Unexamined Patent Application Publication No. 2004-255842    Patent Document 6: Japanese Unexamined Patent Application Publication No. 2002-052842    Patent Document 7: Japanese Unexamined Patent Application Publication No. 2004-237476    Patent Document 8: Japanese Unexamined Patent Application Publication No. 2004-009593    Patent Document 9: Japanese Unexamined Patent Application Publication No. 2005-104134